The invention relates to methods and apparatus for sampling reservoir fluid.
A reservoir is a rock formation in which fluids, e.g., hydrocarbons such as oil and natural gas and water, have accumulated. Due to gravitational forces, the fluids in the reservoir are segregated according to their densities, with the lighter fluid towards the top of the reservoir and the heavier fluid towards the bottom of the reservoir. One of the main objectives of formation testing is to obtain representative samples of the reservoir fluid. Commonly, reservoir fluid is sampled using a formation tester, such as the Modular Formation Dynamics Tester (MDT), available from Schlumberger Technology Corporation, Houston, Tex. In practice, the formation tester is conveyed, generally on the end of a wireline, to a desired depth in a borehole drilled through the formation. The formation tester includes a probe that can be set against the borehole wall, allowing reservoir fluid to be drawn into a flow line in the formation tester. The formation tester also includes a pump and one or more sample chambers. Typically, optical fluid analyzers are inserted into the flow line of the formation tester to monitor the fluid(s) flowing in various locations of interest. For example, an optical analyzer is often run directly above the probe to monitor the type of fluid entering the flow line.
Initially, the fluid drawn into the flow line is a mixture of reservoir fluid and mud filtrate. To obtain a sufficiently high quality fluid sample, a cleanup step in which mud filtrate is purged from the flow line is performed. This step involves pumping fluid through the flow line and out into the well. As pumping continues, more and more of the reservoir fluid is consumed around the inlet of the probe. Eventually, a fluid mixture that is more representative of the reservoir fluid starts to enter the flow line. Optical fluid analyzers are used to monitor the content of the fluid entering the flow line and how the fluid proceeds through the tool. When the mud filtrate content of the fluid entering the flow line is reduced to an acceptable level, the sample chamber is opened and fluid in the flow line is pumped into the sample chamber. Usually, the following ancillary objectives are set for this step: (1) that a certain minimum volume of reservoir fluid be captured, and (2) that the reservoir fluid captured be single hydrocarbon phase, e.g., oil phase or gas phase, but not both. Finally, the sample chamber is closed and returned to the surface.
In practice, there is only a certain maximum time allowed before the cleanup step must be terminated. Therefore, there is no guarantee that the fluid mixture in the flow line is adequately decontaminated prior to being captured in the sample chamber. Further, the sample chamber may be returned to the surface without a sample, e.g., because the sample chamber was not opened successfully. Further, the sample chamber may be returned to the surface unclosed, e.g., because the sample chamber was not successfully closed after the fluid sample was collected. In this case, the content of the sample chamber may be lost or exposed to contaminants or undergo a phase change as it is returned to the surface. Prior to the present invention, the inventors are not aware of methods for verifying in real-time that the three steps described above, i.e., cleanup, sample capture, and sample chamber closing, are successfully accomplished before the sample chamber is retrieved to the surface.
From the foregoing, there is desired a method of assuring quality fluid sample capture from a reservoir.